Increase in nitrate concentrations is observed in ground waters around the globe, mostly resulting from intensive application of fertilizers. Excessive nitrate concentration is the main reason for closure of wells in the coastal aquifer, resulting in loss of substantial amounts of water. For example, in Israel, nitrate concentrations exceeding the 70 mg/L (˜16 mg NO3−—N/L) standard results in an annual water loss of ˜24 million m3. Application of advanced treatment technologies is required to reduce nitrate concentrations from 90-120 or higher to below 60 mg/L in a cost effective and efficient fashion.
Advanced physical-chemical treatment techniques such as reverse osmosis (RO), ion exchange and electrodialysis are known to be effective for removing nitrates. However, all three methods produce waste concentrates (brines) containing high concentrations of nitrate, as well as other ions. Brine disposal is regulated all over the world, mainly because chloride (Cl−) can have negative effects on ecosystems, for example on freshwater organisms, plants and groundwater. In many inland places, local regulations regarding discharge of brines to the sewage system limit the application of physical-chemical technologies. For example, in Israel, because of agricultural concerns associated with irrigation with reclaimed water, chloride concentration in brine disposal is stringently regulated. The threshold sodium (Na+) and chloride (Cl−) concentrations for disposal to the sewage are 230 and 430 mg/L, respectively. In the United States, chloride (Cl−) limitations in general NPDES (National Pollutant Discharge Elimination System) permits vary between 250 to 1,000 mg/l (e.g., Illinois Louisiana, Mo.), while individual NPDES permits can be much more stringent (e.g., 150 mg/l in California and Florida). In addition, NPDES permits for membrane desalination and ion exchange plants may also require limits of chloride loadings (on top of concentration limitation). (Drinking Water Treatment Plant Residuals Management Technical Report. Summary of Residuals Generation, Treatment, and Disposal at Large Community Water Systems. September 2011. EPA 820-R-11-003).
Treatment facilities installed for NO3− removal from drinking water are mainly based on separation by electrodialysis and RO. On top of brine production, another drawback of membrane technologies is the potential for chemical fouling of the membrane. To minimize chemical fouling, operation with low water recovery ratios or the use of chemicals (antiscalants) is practiced. Biological denitrification, the alternative treatment method, does not produce waste brine but requires an intensive post-treatment step to remove potential water contamination by organic matter and bacteria. Moreover, health concerns and public acceptance constraints limit the application of biological treatment of drinking water [1-3].
Nanofiltration (NF) is a promising technology which has been reported suitable for groundwater treatment. It is defined as a process with characteristics between RO and ultrafiltration and comprises a variety of membrane types with different retention efficiencies for either mono- and multivalent ions [4]. The main advantage of NF over RO is operation under lower pressures and higher recoveries [5]. The use of NF technology was extensively reported for water treatment processes as a sole treatment stage [4-10] or in combination with RO [11-13,16]. Rauternbach and Groschl suggested a scheme for nitrate removal based on RO with the addition of NF+Na2SO4 to achieve high water recovery ratio [16]. However, with respect to NO3− removal all of these processes, in their current development stage, are still limited by the aforementioned drawback associated with production of concentrated brines and the accompanied disposal issue.
There is an unmet need for economical and reliable techniques for removal of nitrate from water, which process generates waste water sufficiently low in brine to enable disposal into local sewage systems.